Give this a try (index=asset_index sourcetype=asset_sourcetype)
| fields asset_id, solution_id, vulnerability_id
| join type=left solution_id [search index=solution_index sourcetype=solution_sourcetype | fields solution_id, solution_summary]
| join type=left vulnerability_id [search index=vulnerability_index sourcetype=vulnerability_sourcetype | fields vulnerability_id, title, severity | rename title as vulnerability_title]
| table asset_id solution_summary vulnerability_title severity
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